1. Field of the Invention
The invention relates to an internal combustion engine that performs combustion by introducing an inert gas into a combustion chamber.
2. Description of the Related Art
Conventionally, in an internal combustion engine, such as, for example, a diesel engine, an engine exhaust passage and an engine intake passage are connected by an exhaust gas recirculation (hereinafter referred to as EGR) passage so as to recirculate an exhaust gas, that is, an EGR gas into the engine intake passage via the EGR passage such that generation of nitrogen oxides NOx is prevented. In this case, the EGR gas has a relatively high specific heat and accordingly can absorb a large amount of heat. Hence, the combustion temperature within the combustion chamber decreases as the amount of the EGR gas is increased. In other words, the EGR rate (the EGR gas amount)/(EGR gas amount+intake air amount) is increased. When the combustion temperature is lowered, the amount of nitrogen oxides NOx generated is lowered. Therefore, the higher the EGR rate, the lower the amount of nitrogen oxides NOx that is generated.
As mentioned above, it has been conventionally known that the amount of nitrogen oxides NOx generated can be lowered by increasing the EGR rate. However, in the case where the EGR rate is increased, an amount of soot generated, i.e., smoke, suddenly starts increasing when the EGR rate exceeds a certain limit. In this respect, it has been conventionally considered that the smoke is unlimitedly increased when the EGR rate is further increased. In other words, the EGR rate at which the smoke suddenly starts increasing is regarded as the maximum allowable limit of the EGR rate.
Accordingly, the EGR rate has been conventionally defined to be within a range which does not deviate from the maximum allowable limit. The maximum allowable limit of the EGR rate differs significantly depending on the type of engine and fuel, however, is typically within a range of about 30% to 50%. Therefore, in the conventional diesel engine, the EGR rate is restricted to the range of about 30% to 50% at most.
As mentioned above, since it has been conventionally considered that the EGR rate has the maximum allowable limit, the EGR rate has been defined to be within the range which does not deviate from the maximum allowable limit, such that the amount of smoke generated becomes as least as possible. However, even if the EGR rate is determined so as to reduce the generated amount of nitrogen oxides NOx and smoke to be as least as possible, the reduction of the generation amount of nitrogen oxides NOx and the smoke is limited because a significant amount of nitrogen oxides NOx and smoke are still generated.